1. Field of the Invention
The present invention relates to a semiconductor memory device and semiconductor integrated circuit and, more particularly, to an SRAM (Static Random Access Memory) including a static memory cell.
2. Description of the Related Art
SRAMs are widely used in general LSIs (Large-Scale Integrated circuits). Along with a reduction in size and a reduction in voltage of LSIs, the SRAM is suffering an increase in power consumption by the leakage current in the standby mode.
More specifically, an SRAM cell has two load P-channel MOS (Metal Oxide Semiconductor) transistors and two driving N-channel MOS transistors. The SRAM cell stores data by turning on/off these MOS transistors. In the MOS transistors, as the gate oxide film becomes thinner, a leakage current (gate leakage) which tunnels and flows through a gate oxide film increases. As the threshold voltage reduces upon a reduction of the power supply voltage, a leakage current (subthreshold leakage) in the OFF state also increases.
As a method of reducing these leakage currents, there is proposed a method of controlling a potential supplied to a memory cell array in the standby mode, and relaxing the cell bias (see reference (Y. Takeyama et al., “A Low Leakage SRAM Macro with Replica Cell Biasing Scheme”, 2005 Symposium on VLSI Circuits Digest of Technical Papers, pp. 166-167)).
In an SRAM in which a source potential VSS_Cell in the memory cell array changes to high level in the standby mode as disclosed in the above reference, the cell bias decreases more than in a normal SRAM, degrading the data retention characteristic. Before a chip is shipped as a product, it must be checked whether the SRAM has a sufficient operating margin. More specifically, in a shipping test, a chip of an insufficient margin must be screened by setting the cell bias lower than that in normal use.
However, this SRAM operates to always keep a predetermined cell bias regardless of variations in power supply potential VDD. Even if the external power supply is simply decreased, screening fails.